Beautiful fanart. Your proportions are all great, but you need more work on your understanding of 3d shapes and how they take volume. The legs and arms don't look like they have bones and muscles. so even when the pose and proportions are right, the body itself isnt right, I traced over your drawing and it looks like this, then I did my version:
even mine isn't perfect so we both need work ;v;
here's how it is in lines alone
the cure is imagine the body in 3d simple shaped and draw them all, even the ones hidden behind something. we may not see them but they do take space. The pose of the other arm is awkward. where is it? seems like there is no place for it, it should be resting somewhere but because it's in the back you treated it like it doesn't exist and hid it.
Draw shapes similar to this
from every angle and in every pose you think of, after a hundred of them you will get better understanding. and don't worry this will get fun the more you do it. and for the coloring. the white light is too strong, and it is not reacting with everything right. I think uncle ben once said: "with great lighting comes great shadows.... "
I try to be funny ):
Here's a few tips when coloring:
1- DON'T use complete white or complete blacks unless you really really really really have to, which should be around 0.0001% of the time.
2- shift hues. don't darken a color with the same version of that color but with some more black in it. your object would still look flat. Shift hues A LOT and don't be too scared of it. it will take practice before you get a better sense of colors so expect the results to be terrible at first.
3- Want to make something light? darken what's around it first. Just because a color is ugly doesn't mean to never use it. Without the ugly things in life nothing would look beautiful. Use a wide range of colors and use more greys and blacks and brawns. The contrast makes things interesting.
4- skin has a million colors in it. red, green, blue, everything. Take some portrait photos done by professional photographers (cuz they do lighting best) and sample some colors on the face and put them somewhere separate out of context. You'd be supersized of the colors you'd find.
5- The most important thing is to THINK 3D this will help you locate the shadows as you have none in your photo. there is no light without shadows. 3d objects should block the light from what is behind them.
I hope I didn't ramble too much